Unless you're going to be using up all your remaining hard drive space, installing Windows on your Mac isn't going to affect its performance in the least bit. The two operating systems exist separately from one another, so Windows will have no effect on your Mac side and vice versa.

As for game performance, it really depends on the game. Most games that came out in the last two or three years, you'll probably manage Medium settings. With older games I'm sure you can hit High settings. So definitely playable settings

Don't worry, we're not Apple fanatics here, nobody is going to pontificate about the evils of installing Windows on your Mac. There are plenty of legitimate uses for it and there are lots of us here at Mac-Forums that have Windows installed on our Macs.